Heat oven to 450 degrees.
Trim any excess fat from beef.
Sprinkle with salt, pepper and rosemary.
Tuck thin end under fillet to make it about the same thickness the full length.
Lay bacon slices over meat and tie the fillet with string lengthwise then crosswise as necessary to secure the end you tucked under and the bacon.
Put meat on bottom of shallow roasting pan (not on rack).
Roast in oven 30 minutes, turning once.
Remove from oven.
Reduce oven temperature to 300 degrees.
Remove and discard string and bacon around meat.
Pour wine over and continue roasting (300 degrees) about 30 minutes more, basting often. (This will give you meat that is well browned on the outside and rare in the centre. If you want the meat a little better done, allow about 40 minutes at 300 degrees.)
Heat butter in large heavy skillet when meat is nearly done.
Add mushrooms and cook gently, turning as needed, about 10 minutes or until tender.
Slice meat about 3/4 inch thick and put on hot platter, surround with mushrooms, pour any liquid from pan over and garnish with parsley or watercress.
Serve immediately with Bearnaise Sauce.